This invention relates to transformers, typically distribution transformers, and it has particular relationship to transformers whose cores are composed of laminations of amorphous metal of small thickness. Typically the thickness of the laminations is of the order of 0.001-inch. While this invention is disclosed herein in detail as applied to a transformer having a core of a wound amorphous metal web of small thickness, as to which this invention has unique advantages, it is understood that the adaptation of the principles of this invention to transformers of other types is within the scope of equivalents of this application and of any patent that may issue on or as a result thereof, typically as the range of equivalents is applied in Graver Tank & Mfg. Co. v. Linde Air Products Co. 339 U.S. 605; 70 Supreme Court Reporter 854 (1950) and interpreted in Uniroyal v. Rudkin-Wiley Corp. 5 USPQ 2d 1434 (CAFC 1988).
U.S. Pat. No. 4,709,471 to Milan D. Valencic and Dennis A. Schaffer, assigned to Westinghouse Electric Corporation discloses a method for making the cores of these transformers. U.S. Pat. No. 4,761,630 to Frank H. Grimes and Eugenius Hammack, also assigned to Westinghouse Electric Corporation, discloses structural features of these cores. Valencic teaches that a web of the amorphous metal is wound into a closed spiral core loop or core structure with a window in the center. The turns of the spiral form the laminations of the core. These laminations forming the closed core structure are then cut in successive stepped groups and the closed structure is opened and converted into a U-shaped structure on whose arms the coils of the transformers are telescoped. The arms of the U-shaped structure are then abutted to reconvert the U-shaped structure into a closed structure. Grimes discloses a core as taught by Valencic with lapped steps.
To explain this invention so that those skilled in the art will understand it, it is desirable to discuss Grimes briefly. FIGS. 3 and 4 of Grimes show highly enlarged a fragmentary part of the core as viewed in a direction perpendicular to the edges of the laminations. If FIG. 3 were extended to show the whole core, it would show the laminations in a race-track or circular annular shape. The structure shown is stepped with the steps lapped at their adjoining laminations. For this application, each unit of laminations A, B or C or D, E or F is defined as a step and the steps A, B and C together or D, E and F together are defined as a group. The laminations are typically only 0.001-inch in thickness so that the step F has a thickness of 0.007. The thickness of group DEF is 0.021-inch. Grimes says that there may be as many as thirty laminations in each step. In this case, each group would have a thickness of 0.09-inch. There may also be less than seven laminations in each step; in fact, there may be one lamination in each step.
While the teachings of Valencic and Grimes have made formidable contributions to the transformer art, a problem has been experienced in the construction of the core as taught by Valencic and Grimes. In the reconstitution of the core after the coils are telescoped on its arms, difficulty has been encountered in matching the separated ends of the laminations of the groups so that severed ends of laminations on one side of a group are joined during reassembly to the ends of the same laminations on the opposite side of a group. The small thickness of the groups is a major factor contributing to this problem. In past practice a group of laminations is at times erroneously matched with two opposing groups during reassembly. The error becomes apparent only on completion of the joint reassembled when a spare group of laminations with no matching companion groups remains. For example, with reference to FIG. 3 of Grimes, the twenty-one laminations on the right of group ABC instead of being joined to the laminations on the left of group ABC, are joined to the laminations on the left of group DEF. When this happens the core as completed has unconnected laminations and must be reprocessed. The deficiency is not discovered until after the core is completed. In addition, after the above-described error occurs, an opposing error may occur during the continued reassembly, i.e., the laminations on the right of a group such as DEF are joined to the laminations on the left of a group such as ABC. In this case, the errors are compensating and are not discovered after the core is completed and the core performance is deteriorated.
Another problem which arises in connection with cores in one of whose yokes or legs severable joints exist, has its roots in the principle that among the various factors that govern the power to excite a transformer is the local net transverse cross-sectional area reduction which results from the gaps in the core that form the joint. The net cross-sectional area is reduced and the required exciting power is increased by the gaps in the laminations at the joint. In cores formed of laminations of amorphous metal, this problem is particularly significant because, in use, such cores are operated at inductions very near to that which produces saturation. Appreciable reduction in the net transverse cross-sectional area of a core may result in saturation when the core is used. In a joint containing a large number of gaps, particularly if the gaps are stacked, there is a substantial reduction in net cross-sections area and a tendency for saturation to occur with corresponding adverse effect upon core losses.
Another problem which occurs when a butt-lap-step core joint as taught by Grimes is reassembled arises from the tendency of the laminations to adhere to each other such that more than one group of cut steps may be mismatched and mislocated against a single opposing group. This error is also only discovered after the joint is completely reassembled. The error occurs on the side of the joint where the longest laminations are on top viewing the joint being reassembled positioned horizontally. The shorter laminations of adjacent groups tend to adhere. The opposite side of the core where the longest laminations are at the bottom are "self separating" by groups.
It is an object of this invention to overcome the above-described drawbacks and deficiencies and to provide a method for accomplishing this purpose. Namely, it is an object of the invention to provide a method of making a transformer having a core of amorphous metal, formed by winding into a spiral a plurality of turns or laminations of small thickness, which core has a severable joint that is opened to permit the telescoping of coils and thereafter reclosed by abutting the ends of the severed laminations, in whose practice, the mismatching of abutted laminations during the reclosing shall be precluded.
It is also an object of this invention to provide a method of making a transformer in whose practice the transverse cross-sectional area at overlapping laminations at the a transformer made by the practice of the method.